The present invention relates to a pellet or sheet useful in thermal energy storage and, more particularly, to a pellet or sheet formed from crosslinked polyethylene and having a straight chain crystalline alkyl hydrocarbon therein.
A great deal of interest exists in phase change thermal energy storage systems because of their inherent ability to store and release large quantities of heat to their surrounding environment as temperatures drop below or exceed predetermined levels. These systems are of particular interest in the architectural and building trades where climate control and its concomitant energy consumption is one of the principal considerations in building design and material selection.
A variety of building materials and techniques, including structural elements which incorporate phase change materials, have previously been used to conserve heat or cool and thereby reduce energy costs. For example, phase change materials have been incorporated into concrete such that energy in excess of that necessary to obtain comfort conditions is inherently absorbed and released as required to maintain the comfort range. Thus, in the winter months, phase change materials incorporated into the concrete walls or floors of buildings absorb solar energy during daylight hours and release it to the interior at night as temperatures decrease. In the summer months, the same phase change materials, due to their thermostatic character, conserve coolness by absorbing cool from the night air, and releasing it during the day.
Concrete materials incorporating phase change materials are more desirable than elements which store only sensible heat because they have a higher capacity to store energy, plus they absorb and release a large quantity of energy over a very narrow temperature range.
A phase change material utilizes its latent heat of fusion for thermal storage. The latent heat of fusion is substantially greater than the sensible heat capacity of the material. Stated differently, the amount of energy which a material absorbs upon melting, or releases upon freezing, is much greater than the amount of energy which it absorbs or releases upon increasing or decreasing in temperature 1.degree. C. Upon melting and freezing, per unit weight, a phase change material absorbs and releases substantially more energy than a sensible heat storage material which is heated or cooled to the same temperature range. In contrast to a sensible heat storage material which absorbs and releases energy essentially uniformly over a broad temperature range, a phase change material absorbs and releases a large quantity of energy in the vicinity of its melting/freezing point. In addition to their latent storage capacity, the phase change materials also store and release sensible energy as well. Thus, the latent storage in phase change materials is always augmented to a significant extent by their sensible storage capacity. This advantage is put to good use in buildings where space is at a premium and energy storage and release are required within a very narrow comfort range.
It has long been recognized that an effective phase change material, which could store and release thermal energy within the temperature range of 10.degree.-65.degree. C., and could be economically incorporated into concrete, would have broad utility for many heating and cooling applications including solar passive, bridge deck deicing, etc.
Widespread use of the direct incorporation of phase change materials into concrete has not been achieved because the phase change material adversely affects the physical properties of the concrete. Direct incorporation of phase change materials into concrete reduces the strength properties. Thus, the degree of concrete crosslinking required to achieve optimum physical properties is not obtained in the direct presence of the phase change material.
It has been suggested to encapsulate phase change materials in pellets for incorporation into concrete and the like. U.S. Pat. No. 4,504,402 to Chen teaches an encapsulated phase change material which is prepared by forming a shell about a phase change composition in compacted powder form. These pellets, however, are comparatively expensive to manufacture.